Process and apparatus for increasing the charge density of insulators

ABSTRACT

This invention relates to a process and apparatus for increasing the charge density on the surface of a dielectric material in which electrons and gas ions are produced in the gas space above the surface of the material by means of direct current so that the surface is electrostatically charged. The process is an improvement which comprises producing an electric field of the same polarity above the source which generates the electrons and gas ions, whereby the migration of the electrons and gas ions to the surface of the material is directed.

United States Patent 1 Haas [ PROCESS AND APPARATUS FOR Filed:

INCREASING THE CHARGE DENSITY OF llNSULATORS Inventor: Giinter Haas, Wiesbaden-Biebrich,

' Germany Assignee: Kalle Aktiengesellsc'hait,

Wiesbaden-Biebrich, Germany Dec. 14, 1970 Appl. No.: 97,946

1 Foreign Application Priority Data Dec. 17, 1969 Germany P 19 63 248.5

11.8. C1. 317/262 A, 250/49.5 ZC Int. Cl. H01t 19/00 Field of Search 317/3, 4, 262, A;

250/49.5 ZC, 49.5 GC, 49.5 TC; 118/637 References Cited UNITED STATES PATENTS 11/1970 Roth 250/495 ZC' June 12, 1973 2,588,699 3/1952 Carlson 317/262 A 3,196,270 7/1965 Rosenthal..... 250/495 Tc 3,557,367 .1/1971 Roth 317/262 A 2,868,989 l/l959 Haacke 317/262 A 3,554,161 l/l97l Blanchette 250/495 ZC X Primary Examiner-J. D. Miller Assistant Examiner-Harry E. Moose, Jr. Attorney-James E. Bryan [57] ABSTRACT This invention relates to a process and apparatus for increasing the charge density on the surface of a dielectric material in which electrons and gas ions are produced in the gas space above the surface of the material by means of direct current so that the surface is electrostatically charged. The process is an improvement which comprises producing an electric field of the same polarity above the source which generates the electrons and gas ions, whereby the migration of the electrons and gas ions to the surface of the material is directed.

7 Claims, 4 Drawing Figures 941mm J'JN 1 2191s SHEEI 1 Bf 4 o E FIG.|

INVEN'I'OI GUNTER HAAS n 9M 56 v ATTORNEY PATENTED M I gum -SHEEIUOF4 FIG. 4

PROCESS AND APPARATUS FOR INCREASING THE CHARGE DENSITY F INSULATORS This invention relates to a process for increasing the charge density during electrostatic charging of the surface of electrically nonconductive materials, in particular of plastic materials, and to an apparatus for performing this process.

It has been long known that electrically nonconductive materials, eg webs of plastic films, acquire an electrostatic charge when they are conveyed over rollers, for example. Further, it is known in the art to charge the surface of webs of plastic material by means of electrons and gas ions. From U. S. Pat. No. 3,068,528, for example, a process is known for stretching and transporting organic thermoplastic materials, in

v which, in addition to other machine parts, the web is conducted over a grounded support and an electrode which may be in the form of a taut wire connected to a voltage generator is arranged above the web. Due to the non-uniform electrical field in the vicinity of the strongly curved surface of the taut wire electrode, electrons and gas ions are formed by ionization of the which impart which an electric charge to the surface of the film facing the electrode and thus cause it to adhere to the grounded support, for example, rollers. This treatment serves the purpose of reducing the slip between the moving support and the web of film to be stretched in order to prevent the web surface in contact with the rollers from being damaged by slipping. In some cases, these processes operate quite satisfactorily. They have the disadvantage, however, that, besides migrating to the surface of the web, the electrons and gas ions produced migrate also into the space surrounding the electrode. Thus, a much larger quantity of electrons and gas ions must be produced when a certain charge density on the surface of the insulator is to be achieved than would be necessary if all the electrons and gas ions migrated to the surface. Therefore, only a limited charge of the surface can be obtained by the known process, because, if a higher voltage were applied for the purpose of increasing the number of electrons and gas ions, this would involve the risk of an electric breakdown and consequent damaging of the film surface.

The present invention provides means of increasing the charge density during electrostatic charging of electrically nonconductive materials, while avoiding the above mentioned disadvantages.

This is achieved by a process wherein direct current is used to produce electrons and gas ions in the space above the material in order to electrostatically charge the surface thereof. In order to direct the migration of the electrons and gas ions to the surface of the material, an electric field of the same polarity is produced above the source which generates the electrons and gas ions. The expression direct current" includes also those voltages which include a certain proportion of residual ripple. Due to the fact that the space surrounding the electrode is of the same polarity as the electrons and gas ions generated, they are prevented from migrating into this space and are almost completely directed toward the surface of the material.

The process of the invention has the advantage that considerably less electrons and gas ions must be produced to achieve a charge density on the surface of the material which is comparable to that obtainable by earlier processes, which means that a considerably higher charge can be given to the surface by applying the same voltage to the electrode.

In a special embodiment of the process of the invention, the source which generates the electrons and gas ions is additionally heated. This causes the further advantage that the process can be performed using voltages between 200 V and 5 kV, preferably between 500 V and 1,000 V, which eliminates serious problems in the insulation of the electrode without reducing the number of electrons and gas ions produced. When the source generating the electrons and gas ions, which may be in the form of a taut thin wire, is not heated, the process of the invention is operated at a direct current of 2 to 30 kV, preferably between 10 and 15 kV. Depending upon the polarity of the electrode producing the electrons and gas ions, an electric field of the same polarity is produced above this source. In the following, the source producing this electric field will be designated as a directional electrode. The maximum charge density which can be produced on the surface of the material without damage to the film can be calculated according to the following formula D e, e E[A sec./V cm V/cm] A sec/cm wherein s the dielectric constant of the vacuum,

e the relative dielectric constant of the material,

and

E the dielectric strength of the material, which depends not only upon the kind of material, but also upon its thickness.

The values for e and for E and, when gases are used instead of a vacuum, also the values for e vary with the material used. s for the vacuum and for air can be regarded as practically equal.

The invention relates further to an apparatus for performing the process. It comprises a grounded support on which the nonconductive material rests, and an electrode arranged above the material, which is pro vided with direct current by a voltage generator. The polarity may be either positive or negative. Above the electrode generating the electrons and gas ions, the directional electrode is arranged which consists of an electrically conductive layer connected to a voltage generator of the same polarity and insulated from the other parts of the apparatus.

The electrically conductive layer consists of one or more wires and/or metal strips connected to the voltage generator. Layers, for example metal layers, which are applied to a nonconductive support by vacuumdeposition and/or lamination, also may be used. In a particular embodiment, the directional electrode corresponds in its shape to the electrode generating the electrons and gas ions, which may be of a predetermined shape.

In a preferred embodiment of the invention, a measuring instrument is interposed between the electrically conductive layer and the voltage generator, in order to be able to measure the flow of current and to adjust the directional electrode. In this manner, it can be easily determined whether and to which degree electrons and gas ions flow from the electrode to the directional electrode during electrostatic charging.

In another embodiment of the apparatus, a highly ohmic resistance is interposed between the directional electrode and thereference potential, instead of connecting the directional electrode to a high-voltage gensuch that only as many electrons and gas ions flow away as reach the directional electrode. The resistance to be interposed is in the range of from 20 to Meg Ohm (M0).

In the preferred embodiment, a measuring instrument is interposed between the resistance and the reference potential to measure the flow of current. Advantageously, a variableresistance is provided in order to enable a fast readjustment, for example when the electrode voltage changes. Although the distance between the directional electrode and the electrode, and the distance between the electrode and the surface of the material can be varied within certain limits, it has proved to be of advantage in practice for the distance between the directional electrode and the electrode to be about equal to the distance between the electrode and the material surface. Depending upon the thickness and the feed speed of the material, and the number of electrons and gas ions produced, the distances range from 0.1 to 15 cm., preferably from 1 to 5 cm.

The invention will be further illustrated by reference to the following specific examples.

EXAMPLE 1 A thin steel wire of 0.3 mm diameter was clamped between electrically insulated mountings over a grounded roller and at a distance of cm therefrom. When the voltages stated below were applied to the wire, the following current values were measured:

U 1 J 1 kV mA 9.0 0.34 9.5 0.54 10.0 0.69 10.5 0.90

EXAMPLE 2 The same arrangement was used as in Example 1, except that, above the taut wire and at a distance of mm therefrom, a curved elongated directional electrode of about 2 cm width was clamped in insulated mountings. This directional electrode was electrically connected with a voltmeter and had a resistance to ground of the order of a few 1,000 Meg Ohm. When different voltages were applied to the wire, the following values were measured, U 1 being the voltage at the wire, U 2 the voltage at the directional electrode, and J l the current issuing from the wire. At equal voltages, the current is markedly lower than in Example 1:

u l u 2 J 1 kV kV mA 9.0 4.5 0.145 9.5 5.0 0.285 10.0 5.3 0.4 10.5 5.8 0.575

EXAMPLE 3 The arrangement used was the same as in Example 2, except that voltages of varying magnitudes were additionally applied to the directional electrode. The current values obtained range between those of Example 1 and Example 2, depending on the voltage applied to the directional electrode. The following values were measured:

U l U 2 J 1 kV kV mA 9.0 4.5 0.16 9.5 4.5 0.325 10.0 4.5 0.5 10.5 4.5 0.675 9.0 5.0 0.15 9.5 5.0 0.30 l0.0 5.0 0.45 10.5 5.0 0.65

When operating according to Examples 2 and 3, the frictional forces of webs of plastic material transported on the roller exceeded those measured in Example 1 by about 20 per cent, at comparable currents.

The accompanying drawings are perspective views of four embodiments of the apparatus for performing the process of the invention.

FIG. 1 shows a section 1 of a web of nonconductive material conveyed on a grounded roller 2. An insulated curved directional electrode 3 is arranged above the web. Via an instrument 4 for measuring the current flow, the directional electrode 3 is connected to a voltage generator 5 fed from the electric supply line N. Between the web 1 and the directional electrode 3, there is the insulated electrode 6 which generates the electrons and gas ions and which, in this case, is fed from the electric supply line N via a voltage generator 7. In this device, the electrode is also heated by the heating device 9 via the connecting wires 8. The apparatus is capable of many variations.

If a high-ohmic resistance is used as the flow-off resistance, the high voltage generator 5 is replaced by an appropriate resistance, which may be variable.

FIGS. 2, 3 and 4 show the highly ohmic resistance 5a which is employed instead of a voltage generator 5 in FIG. 1. FIG. 3 shows a conductive layer applied to a dielectric support 3a, and FIG. 4 shows the wire 3a which replaces the directional electrode 3 of FIG. 1.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

1. An apparatus for increasing the charge density on a surface of a dielectric material which comprises grounded support means, electrode means mounted above the support means and connected to a D.C. source, and an electrically conductive layer means above the electrode means and being connected to a reference potential over a highly ohmic resistance and insulated from the remainder of the apparatus.

2. An apparatus according to claim 1 including means for heating the electrode means.

3. An apparatus according to claim 1 including current measuring means between the highly ohmic resistance and the electrically conductive layer.

4. An apparatus according to claim 1 in which the electrically conductive layer means comprises at least one wire.

5. An apparatus according to claim 1 in which the electrically conductive layer means comprises at least one metal strip.

6. An apparatus according to claim 1 in which the electrically conductive layer'means comprises at least one conductive layer applied to a dielectric support.

7. An apparatus according to claim 1 in which the highly ohmic resistance has a resistance of 20 to 00 M0. a :r a 

1. An apparatus for increasing the charge density on a surface of a dielectric material which comprises grounded support means, electrode means mounted above the support means and connected to a D.C. source, and an electrically conductive layer means above the electrode means and being connected to a reference potential over a highly ohmic resistance and insulated from the remainder of the apparatus.
 2. An apparatus according to claim 1 including means for heating the electrode means.
 3. An apparatus according to claim 1 including current measuring means between the highly ohmic resistance and the electrically conductive layer.
 4. An apparatus according to claim 1 in which the electrically conductive layer means comprises at least one wire.
 5. An apparatus according to claim 1 in which the electrically conductive layer means comprises at least one metal strip.
 6. An apparatus according to claim 1 in which the electrically conductive layer means comprises at least one conductive layer applied to a dielectric support.
 7. An apparatus according to claim 1 in which the highly ohmic resistance has a resistance of 20 to Infinity M Omega . 